Various oil formulations have been used to lubricate equipment and reduce component wear. When operation pressures increase in equipment which generates or is subject to high fluid pressures, very tight tolerances must be maintained between the surfaces of sealed dynamic components, and accordingly good lubrication of these surfaces is a significant problem. Moreover, equipment components subject to extremely high operating pressures frequently must be fabricated from expensive materials, and repair or replacement of worn or corroded components is very costly.
One of the most difficult challenges for an equipment lubricant is presented by a hypercompressor of the type commonly used to manufacture polyethylene. Hypercompressors which employ reciprocating solid tungsten carbide or tungsten carbide coated steel rods are commonly used to compress ethylene at pressures in the range of from 35,000 to 45,000 psi to extrude beads of polypropylene. Ethylene gas is commonly sealed within the compressor by bronze tings and packing cups which receive the reciprocating rods. Replacement of these tings and packing cups may result in the shutdown of an entire polyethylene production facility. Accordingly, hypercompressor repairs and service may be very costly, both with respect to the time and expertise required to replace components, and more importantly with respect to the huge investment of polyethylene manufacturing equipment which is inactive during these repair or service operations. Further details with respect to service of a compressor used in polyethylene manufacturing, a flowchart of a typical polyethylene production operation, and lubricants for a polyethylene production compressor are disclosed in the article by Carl W. Wikelski entitled "Lubrication of Compression Cylinders Used in the Manufacture of High-Pressure Polyethylene", Lubrication Engineering, Vol. 37, pps. 203-208 (1980).
Although polyethylene has many uses, it is widely used in packaging and other applications where the polyethylene comes into at least incidental contact with food for human consumption. When manufacturing polyethylene for these applications, food grade lubricants must be used in the hypercompressors since the lubricant could contaminate the polyethylene and thus the food. "White oils" comprising substantially only hydrogen and carbon molecules which are commonly formed by passing hydrocarbons through a hydrogenation unit to remove aromatic groups and other possibly deleterious substances. White oils available from various manufacturers meet the approval of the U.S. Food and Drug Administration (FDA) for incidental food contact. Unfortunately, pure white oil is not an effective lubricant when used in hypercompressors, and accordingly various food grade high lubricity oils, thickeners, antioxidants, and/or catalytic initiators are commonly added to white oil to increase its performance as a lubricant when used in hypercompressors.
Oleic acid has long been added to white oil to increase its performance in hypercompressors. The addition of oleic acid enhances the wear properties of white oil by increasing the lubricant film strength. In the presence of water, however, oleic acid is corrosive on the bronze rings and packing glands used in hypercompressors. Oleic acid and water are also corrosive on the cobalt binder commonly used to adhere the tungsten carbide coating to steel rods of hypercompressors. Nevertheless, oleic acid has long been used as an additive for enhancing the properties of white oils used to lubricate hypercompressors because it is one of the few food grade additives which significantly enhances the lubricity of the white oil.
Recent research has indicated that hypercompressor repairs are frequently not the result of wear on the rings or glands, but rather the result of corrosion of the rings and glands. Corrosion of these components occurs even when relatively low levels of water are present in the oil, typically in the range of less than 200 parts per million. Moisture may be inadvertently added to the white oil through small leaks in the hydraulic system, and unfortunately most polyethylene production plants located in the United States are in the humid southwestern part of the country. Also, hydrogen peroxide is a common catalyst in polyethylene production, and water formed as a by-product in polyethylene production may contaminate the white oil. While various efforts have long been undertaken to reduce the moisture content in white oils used in hypercompressors, water content in excess of 10 parts per million in hypercompressors is nevertheless common, and even low water levels can be highly corrosive on the rings and glands at these high pressure levels.
The disadvantages of the prior art are overcome by the present invention, which significantly reduces or eliminates the use of oleic acid as a lubricity agent to make the white oil suitable for use within hypercompressors. Accordingly, the compressor oil of the present invention results in significantly less corrosion of components yet exhibits extremely low frictional wear.